Low protein diets: what can be achieved?

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There has been much debate recently on the merits of reducing protein levels in dairy cow diets. As long as soya prices remain firm, the prospect of cutting back on protein is certainly attractive. And from the cow’s perspective, feeding excess protein uses up valuable energy supplies that could be utilized more efficiently elsewhere. So what scope is there to remove protein from dairy diets?

Most dairy rations in Northern Ireland supply around 17.5-18.0% crude protein in the total diet. US research however indicates that a protein level closer to 16% is optimum for high yielding cows. So are we feeding too much protein?

To answer this question you first have to consider the basal forage. There is a huge difference between the maize silage-based diets typical of US dairy systems and those of grass silage-based diets in Northern Ireland. Maize silage is typically a high starch, low protein forage so utilization of the rumen degradable protein fraction is relatively efficient. Grass silage, on the other hand, contains up to twice the level of protein found in maize silage and most of this protein (about 60%) is highly soluble. But with less than 6% fermentable sugars, achieving efficient protein utilization with grass silage-based diets is much more difficult.

So are low protein diets a non-runner in Northern Ireland? Not exactly, but simply removing protein is not the answer. In practice, high yielding cows benefit from feeding a little extra high quality protein, such as soyabean meal, because the excess amino acids can be utilized as an energy source. This benefit is often seen in terms of a small increase in milk production, although the extra energy can also help improve milk protein content or replenish body reserves. There is no conclusive evidence that reducing protein supply will improve health and fertility.

Mid-late lactation cows use a greater proportion of their nutrient intake to replenish lost body reserves rather than increase milk production. Research at AFBI Hillsborough has shown that when cows have reached 150 days in milk and are in good condition (BCS >2.5), protein levels can be reduced from 17.3% to as low as 14.4% without impacting on milk production. So managing high yielding and stale cows in separate groups provides some scope to reduce protein inputs.

Before protein levels can be reduced, the challenge is to provide cows with the means to utilize protein from grass silage more efficiently. Achieving the correct balance between slowly fermentable fibre and rapidly fermented starch is key.

A further strategy which McLarnon’s are exploring is to improve the amino acid balance in dairy rations. Dairy cow diets are generally undersupplied with methionine and lysine, which sets an upper limit on milk production, but are oversupplied with most other amino acids. Disposing of the excess amino acids has an energy cost, but supplementing diets with methionine and lysine to obtain a more favourable amino acid balance could enable protein levels to be reduced without impacting on cow performance. On-farm trials are currently on-going to investigate this strategy and its wider impact on cow health, fertility and performance.

If you require help and advice with feeding your dairy cows, feel free to contact Ronald Annett at McLarnon Feeds on 028 7965 0321.